The present invention relates to ink formulations suitable for ink jet printing systems, and more particularly for indirect printing systems. Ink film constructions produced using such inks including ink dots, and more particularly continuous ink dots, adhered to printing substrates are also disclosed.
Currently, lithographic printing is the process in most common use for producing newspapers and magazines. Lithographic printing involves the preparation of plates bearing the image to be printed, which plates are mounted on a plate cylinder. An ink image produced on the plate cylinder is transferred to an offset cylinder that carries a rubber blanket. From the blanket, the image is applied to paper, card or another printing medium, termed the substrate, which is fed between the offset cylinder and an impression cylinder. For a wide variety of well-known reasons, offset litho printing is suitable, and economically viable, only for long print runs.
More recently, digital printing techniques have been developed that allow a printing device to receive instructions directly from a computer without the need to prepare printing plates. Amongst these are color laser printers that use the xerographic process. Color laser printers using dry toners are suitable for certain applications, but they do not produce images of a quality acceptable for publications such as magazines.
A process that is better suited for short run high quality digital printing is used in the HP-Indigo digital printing press. In this process, an electrostatic image is produced on an electrically charged image-bearing cylinder by exposure to laser light. The electrostatic charge attracts oil-based inks to form a color ink image on the image-bearing cylinder. The ink image is then transferred by way of a blanket cylinder onto the substrate. Though such systems are better suited for high quality digital printing, the use of oil-based inks has raised environmental concerns.
Various printing devices have also previously been proposed that use an indirect inkjet printing process, this being a process in which an inkjet print head is used to print an image onto the surface an intermediate transfer member, which is then used to transfer the image onto a substrate. The intermediate transfer member (also called an image transfer member or an ITM) may be a rigid drum or a flexible belt, also herein termed a blanket, guided over rollers.
Using an indirect printing technique overcomes many problems associated with inkjet printing directly onto the substrate. For example, inkjet printing directly onto porous paper, or other fibrous material, results in poor image quality because of variation of the distance between the print head and the surface of the substrate, and because of the substrate acting as a wick. Fibrous substrates, such as paper, generally require specific coatings engineered to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate. Using specially coated substrates is, however, a costly option that is unsuitable for certain printing applications. Furthermore, the use of coated substrates creates its own problems in that the surface of the substrate remains wet and additional costly steps are needed to dry the ink so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. Furthermore, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed perfecting or duplex printing) difficult, if not impossible.
The use of an indirect technique, on the other hand, allows the distance between the image transfer surface and the inkjet print head to be maintained constant, reduces wetting of the substrate as the ink can be dried on the image transfer surface (also termed the release layer) before being applied to the substrate. Consequently, the final image quality of the ink film on the substrate is less affected by the physical properties of the substrate.
Such complex indirect printing systems may operate under numerous sets of inter-related variables, including, among others, the formulation of the inks, the composition of the release layer interfacing therewith, the temperatures at which the inks are deposited, dried and transferred, and the pressure applied on the dried ink image to enable transfer.
While ink formulations have been proposed, and notwithstanding their respective quality in the printing systems they have been adapted to, there remains a need for further improvements in ink formulations suitable for ink jetting, and in particular, ink formulations suitable for ink jetting on the intermediate transfer member of an indirect printing system. Quality ink film constructions are also desired.